Process and apparatus for determining the distribution of particle sizes in ground material

ABSTRACT

The particle size distribution in ground material is determined by bringing into contact with a layer of the ground material a mechanical sensing element which is deflectable with at least one degree of freedom, while a relative movement of predetermined constant velocity between the sensing element and the ground material takes place in a direction transverse to the direction in which the sensing element is deflectable. The resulting mechanical deflections of the sensing element are converted into electrical pulses, each of which is produced by contact of an individual particle of the material with the sensing element. In the resulting train of pulses, the number of pulses whose amplitude exceeds a threshold value which occur during a predetermined measuring interval of time are counted.

United States Patent [72] Inventor Otto Heinemann Eunigerloh, Germany[21] Appl. No. 819,802

[22] Filed Apr. 28, 1969 [45] Patented Dec. 28, I971 [73] AssigneePolysius AG Neubeckum, Germany [54] PROCESS AND APPARATUS FORDETERMINING THE DISTRIBUTION OF PARTICLE SIZES IN Primary Examiner-S.Clement Swisher Atrorney- Marshall & Yeasting ABSTRACT: The particlesize distribution in ground material is determined by bringing intocontact with a layer of the ground material a mechanical sensing elementwhich is deflectable with at least one degree of freedom, while a relative movement of predetermined constant velocity between the sensingelement and the ground material takes place in a direction transverse tothe direction in which the sensing element is deflectable. The resultingmechanical deflections of the sensing element are converted intoelectrical pulses, each of which is produced by contact of an individualparticle of the material with the sensing element. In the resultingtrain of pulses, the number of pulses whose amplitude exceeds athreshold value which occur during a predetermined measuring interval oftime are counted.

A [1 54 //VG 4N0 COMPPESS/NG MfC/l/i N/SM PAHNFEU nics m SHEET 2 UF 3PROCESS AND APPARATUS FOR DETERMINING THE DISTRIBUTION OF PARTICLE SIZESIN GROUND MATERIAL The invention relates to a process as well as anapparatus for determining the particle size distribution in groundmaterial.

In the grinding of material (for example ore or limestone) apredetermined particle size distribution often must be maintained. Thusin general the coarse fractions (for example between 90 and 500 microns)in the finished material must not exceed a predetermined proportion. Onetherefore limits the rated capacity of a grinding installation ingeneral with predetermined residues of coarse material (for example over200 microns and over 90 microns).

The previous methods of determining the particle size distribution inground material (for example by screening of sifting) areextraordinarily time consuming and laborious and usually can be carriedout only by sampling. This then can have the result that a grindinginstallation in the interval between two such samplings may produceinsufficient finished material or may grind inefficiently. Thepreviously known methods of determining the particle size distributionin ground material suffer the further disadvantage that they are poorlyadapted for automation.

The object of the invention therefore is to provide a process whichmakes possible a rapid determination of the particle size distributionin ground material, in a particularly simple manner, and permitsautomatic operation.

In accordance with the invention, this object is achieved by bringing incontact with a layer of the ground material a mechanical sensing elementwhose deflecting movement has at least I of freedom; providing relativemovement of a predetermined constant velocity, between the sensingelement and the ground material, transverse to the direction ofdeflection of the sensing element; converting the mechanical deflectingmovement of the sensing element into electrical current or voltagepulses; and counting, in this chain of pulses, the pulses whoseamplitude exceeds at least one threshold value, which occur during apredetermined measuring interval of time.

As can be demonstrated by extensive comparative tests, in the process ofthe invention the pulse amplitude provides a measurement of the size ofthe particles, which pulse amplitude is exhibited by the pulses inquestion, while the pulse frequency is a measurement of the number ofparticles which come into contact with the sensing element in apredetermined relative movement between the material and the sensingelement.

When various threshold values are established, and the pulses within apredetermined measuring interval of time which exceed the thresholdvalues in question are counted, rather accurate evidence concerning theparticle size distribution in the ground material is obtained. Sincethese analyses of the train of electrical pulses conveniently can becarried out electronically, the process according to the invention isexcellently suited to automation.

Thus it also makes possible an ideal control of a grinding installationin such a manner that the desired particle size distribution can bemaintained. The result which is provided by the process according to theinvention of course can be used also for controlling a screeningapparatus.

In accordance with a preferred embodiment of the invention the groundmaterial is furnished in a thin layer on a belt conveyor which has acontinuous linear movement. Preferably in this case the layer of groundmaterial is leveled and compressed before it contacts the sensingelement, so that independently of the accidentally more or less loosedepositing of the material, uniform conditions are always provided forthe sensing element which is deflected by the individual particles ofthe material.

In a first variant of the process according to the invention, a sensingelement, for example in the form of a pin, penetrates into the layer ofthe ground material and is movable parallel to the surface of the layeras well as transversely to the direction of movement of the conveyorbelt.

In a second alternative of the process according to the invention, thesensing element, for example in the form of a sliding shoe, lies uponthe layer of the ground material and is movable perpendicular to thesurface of the layer as well as transversely to the direction ofmovement of the conveyor belt.

In both cases, the sensing element is more or less strongly andfrequently deflected by the particles of the ground material which moverelative to the sensing element.

The transformation of the mechanical deflecting movement of the sensingelement into electrical current or voltage pulses can take place invarious ways. For this purpose, inductive, capacitive and piezoelectrictransducers in particular are available. Further, it may be advantageousto modulate a carrier frequency current in accordance with thedeflections of the sensing element. The application of the carrierfrequency technique generally facilitates the designing of the amplifierwhich is connected to the output of the sensing element.

It is to be understood that, depending upon the further purpose to whichthe measurements yielded by the process of the present invention are tobe applied, the most varied controlling, regulating and recordingdevices may be provided, in which the measurements obtained as to theparticle size distribution in the ground material may be furtherprocessed.

Two embodiments of the invention are schematically illustrated in thedrawings, in which:

FIG. I is a schematic view of a first embodiment of an apparatus forcarrying out the process according to the invention.

FIG. 2 is a schematic view ofa further embodiment.

FIG. 3 is a diagram of the train of electrical pulses obtained in theprocess according to the invention.

A sample stream is divided from the finished ground material and isintroduced in a thin layer 1 upon a conveyor belt 2 which iscontinuously moved linearly in the direction of the arrow 3.

Above the conveyor belt 2 is provided a sensing element 4, which in theembodiment according to FIG. I is in the form of pin and penetratessomewhat into the layer 1 of the ground material. The layer 1 for thispurpose is leveled and compressed ahead of the zone in which the sensingelement 4 is arranged.

The sensing element 4 is connected to a piezoelectric crystal 5 which,by means of two rubber mounts 6, 7, is cushioned and suspended in such amanner that the sensing element 4 is movable in the direction of thedouble arrow 8, i.e., parallel to the surface of the layer 1 as well astransverse to the direction of movement (arrow 3) of the conveyor belt2.

In the embodiment according to FIG. 2 the sensing element 4' is in theform of a sliding shoe and rests upon the layer 1 of the groundmaterial. The crystal 5 connected with the sensing element 4' issupported in two rubber mounts 6', 7 in such a manner that the sensingelement 4 is movable in the direction of the double arrow 8, i.e.,perpendicular to the surface of the layer 1 as well as transverse to thedirection of movement (arrow 3) of the conveyor belt 2.

The sensing element 4 or 4' is deflected by the individual particles inthe layer 1 of the ground material, in the direction in which it ismovable (arrow 8 or 8). The piezoelectric crystal 5 or 5' transformsthese mechanical movements of the sensing element 4, 4 into electricalvoltage pulses, which are then suitably amplified and finally analyzed.

FIG. 3 shows such a train of pulses. The amplitude of the individualpulse is thus a measurement of the size of the particle which hasproduced the respective deviation of the sensing element, while thefrequency of the pulses is a measurement of the number of particleswhich have contacted the sensing element in a predetermined measuringinterval of time T.

The analysis of the train of pulses for the purpose of determining theparticle size distribution in the ground material then ensues in such amanner that the train of pulses is supplied to several filters,connected for example in parallel, which are adjusted to differentthreshold values (for example, S and S and from time to time transmitonly those pulses whose amplitude exceeds the respective thresholdvalue. Beyond these filters the transmitted pulses are then counted.Thus for example in the measuring interval of time T, the filter withthe threshold value S transmits a total of seven pulses and the filterwith the threshold value S transmits one pulse.

Calibration in the process according to the invention is accomplished bycomparative tests with other processes. Thus the threshold values (forexample S and S are so fixed that they correspond accurately to theparticle sizes (for example 90 and 200 microns) for which the residue isto be determined. With this initial calibration, further therelationship between the number of pulses counted in the measuringinterval of time T and the percentage residue corresponding thereto isdetermined.

When in accordance with the process of the invention the particle sizedistribution (i.e., the percentage residue in accordance with theparticle size) has been determined, then with the air of a calculatedcurve which gives the specific surface (Blaine value in cm."/ g.) as afunction of the particle size, the mean specific surface of the groundmaterial can be computed, if desired automatically by electrical means.

I claim:

1. A process for determining the particle size distribution in groundmaterial, comprising the steps of leveling and compressing a layer ofthe ground material, and then bringing into contact with the layer ofthe ground material a mechanical sensing element which is deflectablewith at least 1 of freedom, producing a relative movement ofpredetermined constant velocity between the sensing element and theground material, transverse to the direction in which the sensingelement is deflectable, converting the resulting mechanical deflectionsof the sensing element into electrical current or voltage pulses, andcounting, from the resulting train of pulses, those pulses whoseamplitude exceeds at least one threshold value which occur during apredetermined measuring interval of time.

2. Apparatus for determining the particle size distribution in groundmaterial, comprising a conveyor for conducting a sample stream of groundmaterial in a uniform linear movement, mechanism for leveling andcompressing the stream of material into a uniform layer on the conveyor,a mechanical sensing element which is in contact with the uniform layeron the conveyor, which extends downward below the level determined bythe leveling and compressing mechanism and thus penetrates into thelayer of material on the conveyor and which is deflectable with at least1 of freedom in a direction parallel to the surface of the layer andtransverse to the direction of movement of the conveyor, and anelectrical transducer for converting the resulting mechanicaldeflections of the sensing elements into pulses.

3. Apparatus according to claim 2 wherein the sensing element is in theform ofa pin.

II! 1K

2. Apparatus for determining the particle size distribution in groundmaterial, comprising a conveyor for conducting a sample stream of groundmaterial in a uniform linear movement, mechanism for leveling andcomprEssing the stream of material into a uniform layer on the conveyor,a mechanical sensing element which is in contact with the uniform layeron the conveyor, which extends downward below the level determined bythe leveling and compressing mechanism and thus penetrates into thelayer of material on the conveyor and which is deflectable with at least1* of freedom in a direction parallel to the surface of the layer andtransverse to the direction of movement of the conveyor, and anelectrical transducer for converting the resulting mechanicaldeflections of the sensing elements into pulses.
 3. Apparatus accordingto claim 2 wherein the sensing element is in the form of a pin.